Cab Switcher (Passive)

[ChrisM]

Got a guy who wants to run the output of this amp into a box that selects between three speaker cabs.

I was thinking it would be as simple as taking one of those 3 position rotary impedance switches we use and wiring it up to switch the hot signal. Connect all the ground of the jacks together. Then just have the switch to select one of the three output (speakers) tips (hot).

Yeah? Or am I missing something?
The only thing I thought I would need to be careful of is making sure the switch is very heavy duty.

[pdf64]

Yes, the Mojo impedance switch would be fine but the user must understand that the switch mustn't be operated while the amp has any kind of a signal going through it - I'm assuming it'll be used with a tube amp. You need some kind of a signed disclaimer that absolves you of liability for any consequential damage, should this misuse occur.
Consider fitting a 220 ohm (8ohm speaker) or 470ohm (16ohm speaker) high power (eg 25 watt bolt to chassis) resistor across the hot input, to mitigate for this happening (so that the amp will always have some kind of a load). Pete.

[selloutrr]

Very heavy duty switch and a make before break contact switch so it's never without load. unless he plans on hitting standby between selecting cabinets. A dummy load or some for of safety load would be advised in the event the switch goes open. even if not the whole wattage something to buy him a quick second to turn off the amp.

sorry Pete that's what i get for speed reading - do as Pete advised

[selloutrr]

...

[selloutrr]

Taking it one step farther you could add a Line Out for plugging directly into mixing desks, etc. I wouldn't use it on solid state amps, or amps bigger then 100Watts though it should be fine up to 200Watts. Tube amps only.

Consider using a cliff jack as the Line out to help prevent confussion.

[ChrisM]

Love it! Thank you sir!

[jjman]

Is the gauge of 600v stranded wire always thick enough to handle the current going to speakers?

[selloutrr]

You can increase the wire to a 1000volts or transformer clippings if you have extras.

[ChrisM]

Selloutrr why the 240R of speaker loss resistance? I would think 8R or 16R would make more sense. Though some load is better than no load. Also you cant hurt a OT with going too high on your load, going too low now that's no good.

The only reason I could think of to use a higher value is it wouldn't burn up as quick with no load compared to a low value (like 8R or 16R).

Ill probably wire two aluminum resistors in parallel and bolt them to the enclosure.

[selloutrr]

They are just to buy you enough time to power off the amp in the event you don't plug it in before you turn on the power. It's not intended to use as a dummy load for running a line out without a cabinet.

[ChrisM]

So that value is kinda arbitrary yeah?

[selloutrr]

sorta. You could remove them all together ... hoping it's never improperly used. basically the transformer when it has no load doesn't put out an exact 4 / 8 / or 16 ohm load. it can arch back on it'self and go high ohm. the resistors revent this and the values are picked based on it preventing the OT from ramping or running wild. If you are always using a fixed ohm load setting (i.e. 8ohms) you can probably use a dummy load 100watt resistor as though it is an airbrake or power brake in MAX suck mode of operation and achive the same result.

[Hellhammer]

I've seen people put these 220-330 ohm resistors right on the secondaries of the OT to protect it from impedance switch problems and other no load and intermittent problems. I think they're that big so they won't affect anything when a low load(speaker) is indeed connected.

[selloutrr]

correct.

[ChrisM]

So I am building this now. It's in a little pedal box, very tight going to use PT cuts for the wiring.


I got two 200R 20W resistors I am going to put in parallel for a 100R 40W speaker protection resistor.
I am going to epoxy it to the chassis. You think this is ok?

The resistor should never heat up unless no load is present (and the resistor actually has to work). Epoxy isn't conductive either as far as I know.